Automatic dishwashing detergent powders typically include substantial amounts of inorganic phosphates, which provide alkalinity and sequester calcium ions. Another important ingredient in powdered autodish detergents is silicate, which functions to prevent corrosion and protect overglaze. It is generally preferred to use liquid silicate solution as the silicate source in such compositions since it tends to be less expensive and is a good agglomerating agent.
There are various types of processes for manufacturing powdered detergent compositions. Among these may be mentioned spray drying, agglomerating, dry blending and hybrids thereof. The manufacture of automatic dishwashing detergents primarily involves agglomeration or dry blending. Agglomeration processes for preparing automatic dishwashing detergents frequently involve spraying the liquid silicates onto mixtures consisting mainly of dry salt ingredients such as sodium tripolyphosphate, chlorinated trisodium phosphate, sodium carbonate, sodium sulfate, sodium chloride, etc.
It is frequently difficult to load the desired amount of liquid silicate onto the salts to be used in the composition. One reason for this is that it may be desirable that the phosphates be prehydrated since phosphates lacking moisture tend to cake in the dishwasher's dispenser cup. In such cases, the presence of water in the phosphates limits the amount of silicate which can be absorbed. The difficulty in loading silicates onto hydrated phosphates tends to result in an uneven distribution of size and formation of fines which give the product an undesirable particle size distribution. In addition, agglomerated products have a tendency to cake in the dispenser cup.
An article by E. J. Schuck and R. E. Temple entitled "Silicated Sodium Carbonate As A Detergent Builder," Proc. Mid-Year Meet., Chem. Spec. Manufac. Assoc. 1972, 58, 82-85 discloses silicated soda ash builders in automatic dishwashing detergents including tripolyphosphate. An example discloses silicated soda ash having a moisture level of 4%. It is not stated that the tripolyphosphate is hydrated. The silicated sodium carbonate is said to give acceptable solubility and good open storage stability.
"Soap/Cosmetics/Chemical Specialties," August, 1987, page 89 discloses a process for making a free-flowing granular non-phosphate machine dishwashing detergent. The process begins by spraying a mixture of surfactant, water and liquid silicate onto soda ash, after which sodium metasilicate is added and mixed continuously.
Temple, U.S. Pat. No. 3,821,119 discloses a method of preparing a particulate detergent builder which includes admixing liquid sodium silicate with anhydrous soda ash, passing the resultant granules through a screen and rapidly heating the screened material to a temperature in excess of 100.degree. C.